1. Field of the Invention:
The invention relates to a simple method of determining the J-value of polyphenylene ethers (PPEs) during the oxidative coupling reaction. The J-value is a measure of the molecular weight of the polymer (see DIN 53 728).
2. Discussion of the Background:
It is well known that the viscosity number does not vary markedly during the early part of the oxidative coupling reaction to form polyphenylene ethers. Only in the last third of the reaction progress does the J-value rise particularly rapidly, namely exponentially. In practice, one is interested in producing a PPE with a molecular weight which is as uniform as possible. Also, one desires a PPE with a given J-value, since PPEs with lower or higher J-values have disadvantages in a given application.
This requires an ability to accurately and rapidly determine the J-value during the oxidative coupling reaction, in order to be able to stop the reaction at the desired termination point. This has been a continuing problem in the production of PPEs.
If one attempts the determination by measuring the amounts of oxygen fed and withdrawn, a measurement which itself is difficult, the result is a very inadequate indication of the course of the reaction. One may not use spectroscopic techniques to follow the reaction either, because the reaction solutions contain catalysts and strongly colored impurities. Further, termination at the desired point requires a measurement of the PPE polycondensation which is capable of being performed in less than one minute.
In general, the use of a viscometer to measure the J-values of polymer solutions is known. DIN 53 728 provides that the solvent be evaporated off, a specified dilute solution be prepared, and the viscosity then be determined in an Ubbelohde viscometer according to DIN 51 562. Such a method is very time-consuming, and therefore unsuitable in the present case. It is also known to follow chemical processes in situ using a viscometer in a flow-through mode. However, this poses the following problems when attempted in connection with the oxidative coupling reaction of phenols:
(1) At the beginning of the reaction the viscosity of the reaction solution is extremely low, being only 1-6 mPa sec, and is not even determinable by an ordinary measuring apparatus;
(2) Many viscometers require specially prepared samples; and
(3) PPEs always deposit to some degree on the walls of the measuring instrument, with major effects on the measurements. Accordingly, normal capillar viscometers are fundamentally unsuitable.
European Pat. No. 0 144 106 discloses a method for determining the viscosity of PPEs during the oxidative coupling reaction. This method consists of intermittently taking samples from the reactor, and terminating the polymerization by adding a liquid. As soon as one has obtained a clear, homogeneous solution, one can measure the transit time of the solution through a capillary tube viscometer, whereby the viscosity, and thereby the molecular weight, is determined by comparison with standard values. This method must be carried out discontinuously, which is a disadvantage. When the viscosity is rising rapidly during the critical end stage of the oxidative coupling reaction, one cannot obtain the instantaneous viscosity number of the reaction solution by this method.
A need continues to exist therefore for a rapid continuous method of determining the J-value during the oxidative coupling reaction of phenols to form polyphenylene ethers.